Small detail problem
This is indeed a very complex geometry to create.


Create the base without the diamondshaped details as a reference.
Create a new mesh and enable Snapping to Surfaces.
Create the diamond shape from the top view. Subdivide it, add supporting edgeloops, then snap it to the reference hilt's surface.
Extrude the remaining loose edges guide them around to the back of the hilt. At this point, I removed three quarters of the geometry and added an XY mirror modifier.
Extrude the concave detail as the last step.